Spindle drive fault- at start up- siemens- bridgeport vmc1000

We have a Bridgeport VMC1000. The machine has Siemens drives and Haidenhain controls. This machine brings up a spindle drive fault when doing the first spindle orientation when switching on the machine. We usually give the spindle a spin by hand and command M3 or M4 and usually it will spin. Once the spindle turns, it never ever gives the fault again. Sometimes we repeat this and repower the machine ones the fault comes up, but once the spindle turns, it keeps on OK. We virtually are not powering off the machine overnight and over weekends, but this is a bit inappropriate. Also, even if we momentarily switch it off after leaving it switched on for weeks it would bring the Spindle drive fault alarm, but once the spindle turns, it will run forever. Any ideas?

I also have a vmc1000 with Siemens drives but dx32 controls that I bought with some issues for super cheap. My main issues were that it would not m19 and threw a drive axis error with any rpm command over 500.

Working with Mark at EMI, we figured out that my spindle encoder belt was heavily worn, spindle drift compensation in the drive was off, and the spindle acceleration rate per second parameter in the control was off.

I guess start there and we'll put our heads together again if needed!

Thanks for your reply. The machine encoder is not driven by belt. The encoder is screwed to the motor and driven directly. Some years ago, we had the same issue and we could not work and we dismantled the encoder and found out that water was finding itself in the chamber into the encoder itself. I have noticed that the machine after being left switched on for some three weeks in a row, came to run properly but we have to see what will happen in a few days time.
It might be that it had some humidity and it dried off, but we will see and post it..